8. Cairo, Egypt: 24.5 million people

7. Karachi, Pakistan: 24.8 million people

Supporters of the political party Pakistan Tehreek-e-Insaf take part in a rally near the mausoleum of Mohammad Ali Jinnah in Karachi, Pakistan, on December 25, 2011.
Akhtar Soomro/REUTERS

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6. Dhaka, Bangladesh: 27.4 million people

People crowd on top of a train entering the Dhaka airport rail station September 24, 2015, as thousands of Bangladeshi Muslims head home to celebrate Eid-al-Adha.Ashikur Rahman/REUTERS

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5. Beijing, China: 27.7 million people

Balloons are released as a crowd looks up during a military parade marking the 70th anniversary of the end of World War II, at Tiananmen Square in Beijing, China on September 3, 2015.REUTERS/Rolex Dela Pena/Pool

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4. Mumbai, India: 27.8 million people

Devotees try to form a human pyramid to break a clay pot containing curd during the celebrations to mark the Hindu festival of Janmashtami in Mumbai, on August 10, 2012.Danish Siddiqui/REUTERS

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3. Shanghai, China: 30.8 million people

Chinese national flags are seen as tourists crowd along the Nanjing Pedestrian Road, a main shopping area, on a national holiday in Shanghai on October 4, 2014.REUTERS/Aly Song

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2. Delhi, India: 36.1 million people

Indian Prime Minister Narendra Modi performs yoga with others to mark the International Day of Yoga, in New Delhi, India, on June 21, 2015.Adnan Abidi/REUTERS

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1. Tokyo, Japan: 37.2 million people

People hold placards and shout slogans as they gather to protest against Japan's Prime Minister Shinzo Abe's security bill outside the parliament in Tokyo on August 30, 2015.REUTERS/Kyodo

The Global Cities Institute projects that Kinshasa, in the Democratic Republic of the Congo, will be the most populous city in the world by 2075, and that by 2100, it will be surpassed by the 88 million future residents of Lagos, Nigeria.